1. Field of Invention
The present invention relates to a low temperature polysilicon (LTPS) thin film transistor (TFT) and method of manufacturing the same. More particularly, the present invention relates to a highly flexible manufacturing of fabricating low temperature polysilicon thin film transistors.
2. Description of Related Art
An outcome of the rapid progress in high-tech products is the popularity of video products such as digital video or imaging devices in our daily life. To be useful, these digital video and imaging devices must provide a high-quality display so that a user can operate a controlling device after reading some important information disseminated from the display.
Thin film transistors are the principle driving devices inside a liquid crystal display (LCD). With a flat and stream-line design, LCD has become one of the most popular vertically erected desktop monitors serving personal computers and game machines. Among the thin film transistor liquid crystal displays, one type of display has internal thin film transistors fabricated using a polysilicon technique that can reduce electron mobility relative to a thin film transistor fabricated using the conventional amorphous silicon technique. In general, a thin film transistor having higher electron mobility can have a smaller dimension and a larger aperture ratio so that the display is able to attain a higher brightness level and consume less power. Furthermore, the increase in electron mobility also permits the fabrication of part of the driving circuits and the thin film transistors together on a glass substrate to improve the performance and reliability of the liquid crystal display panel. Therefore, overall cost of producing the liquid crystal display panel is lowered considerably compared with the amorphous silicon thin film transistor liquid crystal display. In addition, since the polysilicon can be fabricated into light and thin sheets of material, it finds many applications in lightweight and low-power portable equipment. However, the conventional method of annealing the amorphous silicon to transform the amorphous silicon into polysilicon demands a temperature of over 600° C. Hence, the substrate must be fabricated using heat-resistant quartz material. Yet, a quartz substrate not only is more expensive than a glass substrate, but a quartz substrate having a linear dimension greater than 2 to 3 inches is also difficult to produce. Because of such limitations, polysilicon thin film transistor panels are formed in relatively small display panels in the past.
To lower production cost, glass is the preferred material for forming the substrate. When a glass substrate is used, the polysilicon layer within the thin film transistors must be produced at a temperature below 500° C. A number of methods for annealing the amorphous silicon at a lower temperature have been developed. One of the most convenient and widely adopted methods is laser annealing. Laser annealing is capable of producing high quality, contamination-free and low-defect-density polysilicon layer. These polysilicon thin film transistors fabricated at a relatively low annealing temperature are frequently referred to as “low temperature thin film transistors.”
At present, the threshold voltage of the low temperature polysilicon thin film transistors is adjusted by performing an ion implantation or an ion shower process. However, both processes demand an ion implantation apparatus. Thus, flexibility in the manufacturing polysilicon thin film transistors is restricted.